A likely function for actin in non muscle cells is to interact with the plasma membrane to transmit tension for cellular movement. Plasma membrane ghosts from malignant human cells in tissue culture (HeLa cells) will be used as a model to study this interaction in vitro. Tubulin, the subunit of microtubules, can be purified and separated by electrophoresis into as many as 4 or 5 closely related components, but the function of these components is poorly understood. Assemblies composed of specific cytoplasmic microtubules will be isolated from HeLa cells and their components of tubulin identified to decide whether specific microtubules are composed of specific components of tubulin. Regulation of actin and tubulin by the HeLa cell will be explored by investigating variations in the concentrations, intracellular distribution, and biosynthesis of actin and tubulin during the cell cycle. It is expected that these studies will advance our understanding of the processes of cytokinesis (where actin appears to interact with the plasma membrane to cause cytoplasmic cleavage) and mitosis (where tubulin forms a structural component of the mitotic apparatus), processes which must be understood in order to understand the growth of normal and malignant cells.